For some time, port liners have been used in exhaust passages of internal combustion engines to minimize cooling of the exhaust gases as they pass from the combustion chamber to the emisson control equipment. It is common knowledge to those skilled in the engine arts that if the temperature of the exhaust gases can be maintained at a high level, then the efficiency of a turbocharger, a catalytic converter or a thermal reactor can be increased. Such an increase improves the fuel efficiency of the vehicle as well as minimizing the quantities of unburned hydrocarbons and carbon monoxide which will be discharged into the atmosphere. Various approaches to reduce the heat loss in the exhaust gases of an engine were presented in the Oct. 18-22, 1976 publication by the Society of Automotive Engineers, pages 1-15. The publication is entitled "An Analytical Study of Exhaust Gas Heat Loss in a Piston Engine Exhaust Port" and was prepared by S. D. Hires and G. L. Pochmara. This paper describes several types of liners including an air gap exhaust port (see page 3, FIG. 7) which can improve the effectiveness of an engine's emission control system by conserving the thermal energy in the exhaust gas. The air gap serves as an insulative barrier which reduces the thermal conductivity of the heat in the exhaust gas as it leaves the combustion chamber of the engine. However, the exhaust port liner depicted in FIG. 7 of the SAE publication is complicated in construction and does not lend itself to assembly line fabrication and assembly. Furthermore, the liner directly contacts the inner periphery of the exhaust passage at several locations thereby producing a segmented air insulating layer.